Growth-sector dependence of optical features in large synthetic diamonds

Abstract

Infrared, ultraviolet and cathodoluminescence measurements have been made on five synthetic diamonds which were grown using the temperature-gradient technique. The samples, which were a few millimeters in size, were chosen by virtue of having relatively well-developed {110} and {113} minor facets as well as {100} and {111} major faces. The samples, labelled I–V, were prepared in the form of polished plates. Sample I was a standard type Ib synthetic diamond. Infrared and ultraviolet absorption measurements were used to determine the concentration of isolated substitutional nitrogen present in the various types of growth sector. The {111} sectors contained about 100 ppm of nitrogen; the {100} sectors contained slightly less than 50 ppm; the {113} sectors contained of the order of 10 ppm and the {110} sectors contained approximately 1 ppm. For the remaining samples, II–V, the concentration of nitrogen incorporated during growth was greatly reduced by the introduction of nitrogen getters into the synthesis capsule. Even so, concentrations of a few ppm of nitrogen could generally be detected in {111} sectors. Sample II contained a concentration of 0.27 ppm of neutral boron acceptors in the (11¯0) sector despite the fact that boron had not been deliberately introduced during synthesis. The remaining three samples were deliberately doped with boron, the amount being less for sample III than for the samples IV and V. For sample III, the uncompensated acceptor concentration was highest in the {110} sectors, being approximately 3 ppm, approximately 0.9 ppm in the {113} sectors, 0.5 ppm in the {100} sectors and too small to be measured in the {111} sectors. For the highly-doped samples, IV and V, however, the uncompensated boron concentration was highest for the {111} sectors. Acceptor-centre zero-phonon lines are broadened out in {111} sectors, presumably as a result of higher defect concentrations in these sectors. Cathodoluminescence measurements at a temperature of approximately 90 K have revealed a new vibronic system with a zero-phonon line at 2.296 eV (539.9 nm). This system was observed only in the (11¯0) sector of sample I and in non-octahedral sectors of the other samples. The 2.56 eV (484 nm) cathodoluminescence system was present with high intensity in all {111} sectors of sample I but only in one {111} sector of sample II. It was absent from sample III and weak in sample IV. All samples exhibited spectrally-broad cathodoluminescence which consisted, in general, of two components, a blue band with a peak at 420 nm and a broader green band with a peak occuring between 490 and 540 nm.